We propose to refine and extend stable isotope (13C) methodology to the study of carbohydrate absorption, especially as it relates to developmental aspects of intestinal function in the premature and term infant. The range of natural fluctuations of 13C/12C ratio in breath, stool and the dialyzable and non-dialyzable fractions thereof must first be determined in premature and full-term infants. This will be done initially in the fasting state and then with unlabelled lactose-containing modified cow milk formula diets. Five groups of infants will be studied. Three groups of premature infants (26-28 weeks, 30-32 weeks, 34-36 weeks gestational age) will be studied sequentially on a weekly basis using 13C-lactose in a dose of 25 mg/kg as part of a single 8 a.m. feeding. Term infants 3-10 days and 3-6 months of age will be similarly studied. Breath H2 excretion, breath 13CO2/12CO2 excretion by mass spectroscopy, and 13C/12C ratio in stool and its subfractions, as well as fecal lactic and volatile fatty acids, will be done to determine when lactose activity develops in the premature infants and what role the colonic microflora plays in lactose digestion when enzyme activity is low. Implicit in all studies is the testing of the hypothesis that all infants with low intestinal lactase activity, especially the premature, rely to a variable degree on their colonic microflora for the digestion of lactose.